Method for making lead arsenate in finely divided form



Patented Feb. 15, 1938' PAT-TENT OFFICE 2,108,553 METHOD FOR. MAKINGLEADARS ENAT IN V FINELY DIVIDED FORM Victor E. Speas and Nathan M. MnookimKansas City, Mo., assignors to Speas Mfg. 00., a corporation of Missouri No Drawing. Application April 22, 1936,

Serial No. 75,750

, 12 Claims. (o1. 23- 54) The present invention relates to methods of preparing lead arsenate and lead arsenate compounds in finely divided form and more particularly to methods of precipitating lead arsenate in afinely divided form from acid solutions of the metaphosphoric or thepyrophosphoricacid compounds containing the same (in a finely dispersed form) It will be fully understood from the following description, in which various specific embodiments of the invention are set forth as illustrative thereof.

The dissolution of lead arsenateby acids and alkali in either dilute or concentrated form has been long known. In general, dilute acids such as hydrochloric acid being preferred. The solvent capacity of these solvents for lead arsenate is comparatively low and experiments have shown the solvent power of such acids as for example hydrochloric acid is directly dependent upon the concentrations of the solvent solutions. In addition, it has been found that the effectiveness of these solvents is increased only with an increase in temperature and the solutions have a tendency to easily become supersaturated and consequently redeposit the 'di ssolved lead compounds in a form not suficiently fine for certain desired purposes as for'example in horticultural sprays. I In accordance with the present invention, it has been found that an effective solvent action upon lead arsenate and lead arsenate compounds may be secured by means of solutions of compounds of the anhydride phosphoric acids, that is, of pyrophosphoric and the metaphosphoric acids, and more particularly of the acids and slightly acidified solutions of their salts, and of the'acidified soluble trimeta and hexametaphosphates. For example, to effect dissolution of the lead compounds, such as lead arsenate, dilute aqueous solutions of pyrophosphoric acid or metaphosphoric acid may be effectively employed. Of the compounds of pyrophosphoric acid and metaphosphorio acids, those of the latter are the more effective and are preferred. While solutions of any desired concentration may be used, it has been found that solutions having concentrations as low'as 0.1 to 1.0% are effective to dissolve these lead compounds. The solutions embodying our invention are'preferably used cold since, in the case of acids, solubility of the lead arsenate'compounds is notincre'ase'd and may in fact,'be decreased at higher temperatures. I In the use of 'such solutions, there is ap-parently a relation between the amount of lead arsenate dissolved and the amount of the effective acid present in the solution, which is'largely independent of the'co'nce'ntration of the latter. Thus, 1% solutions of metaphosphoric acid, freshly preparedby dissolving the proper quantity of P205 in water, at ordinary temperature (say 60 to 80 F) dissolves about 1 gram 'of lead arsenate for eaoh'gram'of metaphosphoric acid present; similarly prepared aqueous solutions containing 05% metaphosphoric acid and 0.2% metaphosphoric acid dissolves substantially the same relative proportions of lead arsenate and a 0.1% solution of metaphosphoric acid dissolves only very slightly less lead arsenate for an equal weight of metaphosphoric acid present. Freshly prepared pyrophosphoric acid acts in a similar manner, although its solvent action is somewhat less; thus solutions of pyrophosphoric acid containing from 0.1% to 1% of pyrophosphoric dissolves less arsenate to an amount from one-third to fourtenths of the amountof pyrophosphoric acid present.

. When warmed, say to temperatures above 170 R, such solutions of metaphosphoric and pyrophosphoric acids dissolve somewhat less than the amounts of lead arsenate that they will dissolve at ordinary temperatures or at temperatures up to 170 F.

The solutions of the lead arsenate compounds prepared as above described may be then neutralized or rendered slightly alkaline to precipitate the lead arsenate compounds in a finely divided form. Any of the conventional and well-known neutralizing agents may be used as for example, sodium hydroxide, potassium hydroxide and the mono-, diandtri-ethanol amines. It is preferred that just sufficient alkali be introduced into' the acid solutions of the lead compounds to exactly neutralize the solutions; however, if desired, these solutions may be rendered slightly alkaline. In no event should sufficient alkali be added to the solutions to effect redissolution of the precipitated lead compounds.

We have found that such solutions of metaphosphoric and-pyrophosphoric acids tend to lose their effectiveness as solvent agents for lead arsenate and lead arsenate compounds if permitted to" stand for substantial lengths of time; for example, the solvent action of a solution of metaphosphoric acid may decrease in one weeks time to about one-third that of the freshly prepared solution. 'While various theories may be offered for this phenomenon, we believe that the comparatively unstable metaphosphoric and pyrophosphoric acids tend to degenerate into the more stable orthophosphoric. acid. In any event, we have made use of this discovery for the preparation of lead arsenate compounds in finely divided form. Thus, we dissolve lead arsenate or its compounds in the metaphosphoric or pyrophosphoric acids as described above, and permit the solution to stand for a time suflicient to permit the precipitation of substantially all of the dissolved lead compounds from the solution. While the time interval for accomplishing this desired precipitation from; the metaphosphoric and pyrophosphoric acids may vary depending upon temperature, concentration, and the like, we have found that a substantially complete precipitation of the desired lead compounds in highly dispersed form will take place from these solutions in about one to two weeks.

The same result may be attained more quickly by heating the solution of the acids described above. Thus for example, lead arsenate in metaphosphoric acid solutions may also be caused to deposit lead arsenate in highly dispersed form by heating the solutions, the solvent actionof the metaphosphoric acid being destroyed as it passes over to the ortho form.

In accordance with the present invention, we have also found that effective stable solutions which closely approach in effective solvent action the prepared acids, may be prepared by using the salts of metaphosphoric and pyrophosphoric acids. The solutions of the salts are preferably slightly acidulated to hasten hydrolysis thereof and to liberate the free acids. Thus, suitable solutions of the soluble salts of metaphosphoric acid, to which sufiicient acid is added to make the solution acid to methyl orange, have been found to be highly effective solvents for lead arsenate and lead arsenate compounds.

For this purpose, there have been employed, in accordance with the present invention, both the soluble inorganic and organic salts of metaphosphoric and pyrophosporic acids; such as the sodium, potassium, ammonium, and ethyl amine salts. These salt solutions are made slightly acid to methyl orange, using any suitable organic or inorganic acid or acid salt for the purpose, such as hydrochloric acid, sulphuric acid, nitric acid, acetic acid, tartaric acid, hydrofluoric acid, picric acid, sodium acid phosphate, sodium acid sulfate or the like. These salts may be used in acid solutions in any desired concentration, and We have found that concentrations as low as 0.1% to 1% were satisfactory. Such mildly acidified solutions, like the solutions of the acids, have a solvent action upon lead arsenate and lead arsenate compounds that appears to depend largely upon the amount of the metaphosphate or pyrophosphate present. Thus, such an aqueous solution of sodium metaphosphate containing 1% of sodium metaphosphate will dissolve an amount of lead arsenate 0.8 times the amount of sodium metaphosphate present; a 0. 5% solution of the sodium metaphosphate prepared in this manner will dissolve an amount of lead arsenate approximately 0.8 times the amount of sodium meta.- phosphate present and a 0.25% solution thus prepared dissolves an amount of lead approximately 0.7 times the amount of sodium metaphosphate present. Unlike the acid solutions, the slightly acidulated salt solutions are comparatively stable. With heating, in the case of the acidulated solutions of the salts, a slight increase in solvent action on lead arsenate and lead arsenate compounds is found. V I

The action of pyrophosphate salts in solution, mildly acidulated, is similar to that of the metaphosphate salts, although to a less degree corresponding to the dilference in the solubility of lead arsenate and lead arsenate compounds in solutions of the acids.

The effectiveness of the soluble hexametaphosphates as for example the sodium, potassium and like hexametaphosphates in mildly acidulated solutions, on lead arsenate compounds, is exemplified by the results obtained with sodium hexametaphosphate as follows:

(1) .01N hydrochloric acid solution dissolved 12.8 mg. in 100 cc.

1% hexametaphosphate plus .01N hydrochloric acid solution dissolved 270 mg. in 100 cc.

.02N hydrochloric acid solution dissolved 18.6 mg. in 100 cc.

1% hexametaphosphate plus .02N hydrochloric acid solution dissolved 420 mg. in 100 cc.

.lON hydrochloric acid solution dissolved 155 mg. in 100 cc.

1% hexametaphosphate plus .10N hydrochloric acid solution dissolved 660 mg. in 100 cc.

It is apparent from the above that a slight acidulation of this metaphosphoric acid compound effects a marked increase in the solubility of the lead arsenate compounds in this solution over that obtained when the hydrochloric acid is used per se.

The solubility of other water soluble metaphosphate salts, as for example sodium trimetaphosphate, for lead arsenate compounds, has been noted; however, they dissolve lead arsenate to a less degree than the water soluble hexametaphosphates.

The solutions of the lead compounds in these phosphate salts are neutralized or rendered slightly alkaline as described above, in order to precipitate the lead compounds, for example, lead arsenate, in a finely divided form.

The finely divided lead arsenate compounds are, as set forth above, suitable for use in sprays. Thus, in the preparation of a spray, a concentrated solution of lead arsenate in an acid solution of a metaphosphoric or pyrophosphoric acid or salt thereof, or a water soluble hexametaphosphate or trimetaphosphate may be neutralized if desired in the presence of other spray constituents, thereby precipitating and making available the lead arsenate in finely divided form in the spray material. 7

By metaphosphoric acid compounds we mean not only metaphosphoric acid and its salts, but also the poly meta compounds such as the trimetaphosphates and the hexametaphosphates.

The present application is a continuation-inpart of our copending application Serial No. 757,416, filed December 13, 1934.

We claim: 1

l. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution of the meta- .phosphoric acid compounds, and neutralizing said solution, thereby precipitating lead arsenate in dispersed form.

2. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution containing an anhydride phosphoric acid compound of the class consisting of hexametaphosphates, and metaphosphoric and pyrophospho-ric acids and their salts; and neutralizing said solution, thereby precipitating lead arsenate in dispersed. form.

3. The method of producing lead arsenate'in highly dispersed form which comprises dissolving lead arsenate in an acid solution of metaphosphoric acid or its salts, and neutralizing said solution, thereby precipitating lead arsenate in dispersed form. V

4. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution of pyrophosphoric acid or its salts, and. neutralizing said solution, thereby precipitating lead arsenate in dispersed form.

5. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution of a soluble hexametaphosphate, and neutralizing said solution, thereby precipitating lead arsenate in dispersed form.

6. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution of sodium hexametaphosphate, and neutralizing said solution, thereby precipitating lead arsensate in dispersed form.

7. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution of a metaphosphoric acid salt, warming said solution, and neutralizing said solution, thereby precipitating lead arsenate in dispersed form.

8. The method of producing sprays containing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in an acid solution of metaphosphoricacid or its salts in the presence of spray constituents, and neutralizing said solution, thereby precipitating lead arsenate in dispersed form in the spray constituents,

9. The method of producing lead arsenate in highly dispersed'form which comprises dissolving lead arsenate in metaphosphoric acid and Warming the solution, thereby precipitating lead arsenate in dispersed form.

10. The method of producing lead arsenate in highly dispersed form which comprises dissolving lead arsenate in metaphosphoric acid and permitting the solution to stand until the acid goes over to the ortho form, thereby precipitating lead arsenate in dispersed form.

11. The method of producing lead arsenate in highly dispersed form which comprises acidifying a solution of a metaphosphoric acid salt with suificient acid to render the solution acid to methyl orange, contacting the solution with lead arsenate to dissolve the latter therein, and neutralizing said acidified solution, thereby precipitating lead arsenate in dispersed form.

12. The method of producing lead arsenate in highly dispersed form which comprises acidifying a solution of sodium hexametaphosphate with sufficient acid to render the solution acid to methyl orange, contacting the solution with lead arsenate to dissolve the latter therein, and neutralizing said acidified solution, thereby precipitating lead arsenate in dispersed form.

. VICTOR E. SPEAS.

NATHAN M. MNOOKIN. 

